Semiconductor packaging involves a variety of processes that require the formation of protective coatings, adhesive layers, bonding layers, pads, and bumps for die attachment applications as well as board level packaging. For example, in semiconductor packaging, a single semiconductor die is typically mounted within a sealed package. In general, the package protects the die from damage and from contaminants in the surrounding atmosphere. The package also provides a system for connecting the electrical devices formed on the die to a printed circuit board or other external circuitry.
The packaging process involves adapting a leadframe (i.e., a metal frame which supports several dies for packaging and provides the leads for the final semiconductor package) to mount several semiconductor dice. Each die has a lower surface (i.e., the back of the die) that is devoid of circuitry, and an upper surface (i.e., the face of the die) having integrated circuitry constructed thereon. The integrated circuitry is electrically accessible via bonding pads which may be arranged in a variety of configurations on the face or edges of the die. During conventional packaging processes, each die is attached to a mounting paddle of the leadframe by an adhesive layer. Adhesives which have been used for this include epoxy, acrylic, silicone, and polyimide materials which are sandwiched between the bottom of the die and the mounting paddle.
During the packaging process, the bond pads formed on the die are electrically connected to the leads of the leadframe using fine bond wires. Following the application of a polyimide protective layer to the face of the die, it, and a portion of the leadframe to which it is attached, is encapsulated in a plastic material, as are all other die/leadframe assemblies on the leadframe strip.
After encapsulation, a trim and form operation separates the resultant interconnected packages and bends the leads of each package into the proper configuration. There is a need in the art for improved methods of attaching semiconductor dies to leadframes and for attaching dies to a printed circuit board.
Recent advances in semiconductor manufacture have lead to higher circuit densities and improved packaging technologies. Some high density circuits use a lead-on-chip (LOC) packaging technology. In general, an LOC die is formed without a mounting paddle for the die. The lead fingers of the leadframe not only electrically attach to the bond pads via the bond wires but also adhere to the face of the die and support it during the encapsulation process. Polymers typically are used as the adhesive for the die and for the necessary structural support.
In addition to the attachment of leads, there is the attachment of dies to printed circuit boards. Chip-on-board (COB) is a term referring to the direct attachment of bare chips onto printed circuit boards (PCBs) by die attach, tape automated bonding (TAB), or flip chip methods. TAB is a technique which allows automation of the bonding of one end of a lead to a semiconductor chip and the other end directly to a printed circuit board. TAB is characterized by formation of a conductive bonding projection or "bump" between the chip and the lead. The bump provides the necessary bonding as well as a physical standoff, which prevents lead/chip shorting. Typically the bumps comprise metals, such as gold alloys.
For flip chip applications, one or more beads of solder (i.e., a bump) are applied to the surface of a die (i.e., that surface having electronic circuitry) and the chip is then flipped over onto the surface of a PCB. Interspersed between bumps is underfill material designed to seal the area between the die and the PCB. Bump dams are also used on flip chips to prevent solder flow and electrical shorting.
Solder paste is a common interconnecting material for packaging applications. Solder paste is a mixture of fusible metal powder, fluxes, activators, solvents, binders, etc. Solder paste is thick and tacky, allowing parts to be held in position without additional adhesives before permanent electrically conductive bonds are formed in the solder reflow process. Typically, solder is delivered to the surface in a desired pattern by screen printing and metal stencil printing. The use of solder paste can be undesirable because the surface can be difficult to clean and because of the presence of heavy metals, such as lead, in the solder.
Other materials used for attaching and electrically connecting parts in semiconductor fabrication, particularly semiconductor packaging, include heat activated adhesive tapes, thermoset adhesives (typically cured after delivery to the surface), and solutions of thermoplastic adhesives.
Adhesive tapes are cut to desired sizes and shapes and applied to a surface. For example, U.S. Pat. No. 4,906,314 (Farnworth et al) describes a method of die attach wherein a polymer coated tape is applied to a surface and heat activated. This method is limited to the shapes and sizes which can be cut from such tape.
Adhesives can also be applied by printing methods (e.g., screen printing and stencil printing). For example, U.S. Pat. No. 5,220,724 (Gerstner) describes the use of a reworkable thermoplastic adhesive to mount surface-mounted devices on a substrate. The adhesive is warmed to make it fluid and is applied to a surface by screen printing. U.S. Pat. No. 5,286,679 (Fanworth, et al) describes a method of attaching a die to the lead fingers of a leadframe using either thermoplastic or thermoset adhesive. The adhesive is dried (if thermoplastic) or cured (if thermoset) after application. The adhesive layer is patterned by hot or cold screen printing, by photopatterning a photosensitive adhesive, or by utilizing a resist method of etch back. However, with these methods, adhesive application can be difficult because the proper viscosity and rheology of the liquid adhesive must be maintained in a narrow range to work properly. In addition, the thickness of the adhesive layer (followed by curing) can be undesirably low for some applications.
Thus, there is also a need for improved methods of forming adhesive layers, bumps, and pads on the surface of dies, leadframes, and printed circuit boards. There is also a need in the art to conveniently deposit materials by screen or stencil printing in semiconductor packaging applications, where such materials would not damage the screen or reduce its working life. There is also a need to develop reliable bonding materials which provide good performance properties as well as exhibit economical processing features.